1. Field of the Invention
The present invention relates to an information recording medium, and a readout method and readout apparatus therefor.
2. Discussion of Background
For information recording media for high density recording by means of lights, it has been common to employ a transparent substrate such as a polycarbonate resin and to apply a light to a writing layer through the substrate.
In recent years, an attempt has been made to obtain an ultra high density medium by a combination of a hard disk technology and an optical disk technology. For example, an optical system is formed on a flying head, and a light is irradiated from the writing layer side to carry out readout of information. This system is called a layer side incident system as opposed to the conventional substrate side incident system. Types of media include a ROM (read only memory) type whereby a change in reflectivity by pits formed on the substrate is readout, a write once type whereby a change in reflectivity is obtained by deforming or modifying a dye or a substrate or by forming pits by heating a layer, a magneto-optical recording type whereby a rotation of a linearly polarized light by a vertical magnetization layer is readout, and a phase change recording type whereby a change in reflectivity between a crystalline state and an amorphous state is readout.
A merit of the layer side incident system as compared with the substrate side incident system may firstly be that good signal characteristics can be obtained, as the incident light is not susceptible to a distortion by the birefringence of the substrate. Further, as compared with a case where the light is transmitted through the substrate, the aberration of light due to an inclination of the disk is small, and the margin against the inclination will be large.
For the same reason, NA (numerical aperture) of the objective lens can be increased, and accordingly, the beam spot can be made small, which makes high densification possible. By bringing the distance between the writing layer and the objective lens to a level of a few tens nm, near field optical recording by a so-called evanescent light will be possible.
On the other hand, with conventional hard disks, designation of an address for an information recording site or formation of a magnetic domain for generation of a servo signal has been carried out for every sheet, which has added to the cost for the production.
Under these circumstances, it has been studied to combine an optical disk technology and a hard disk technology to reduce the production cost to a large extent.
Namely, it has been proposed that a substrate is made of a resin, and such an address signal or a servo signal is embedded in the form of recesses (pits or grooves, which are hereinafter represented by pits) in the substrate at the same time as the resin substrate is prepared.
Such signals by pits in the substrate (hereinafter referred to as ROM signals) will be readout by applying a light to the substrate surface and detecting a change in reflectivity due to the light interference.
As a medium of the layer side incident system, a medium having servo sample pits formed, is disclosed, for example, in U.S. Pat. No. 5,202,880.
However, according to a study by the present inventors, it has been found that signals can hardly be obtainable by the layer side incident system, when the above mentioned pits are recorded in a high density. Namely, even with pits of a density at which sufficiently large signal amplitudes can be taken in the case of readout from the substrate side, only slight signals can be obtained in the case where readout is carried out by the layer side incident system. Especially when the pit width becomes narrow along the high densification, the deterioration in readout signals is remarkable as compared with the substrate side incident system.